Aspects of the exemplary embodiment relate to the purification of spinel-containing powders and find particular application in connection with the formation of low absorption spinel ceramics.
High-energy laser (HEL) exit aperture (windows) are currently being evaluated for various land, sea, and air based platforms. Some of these systems operate in or have to withstand harsh environment of sand storm, hurricane, and rain. Ideal exit aperture windows for HEL systems should possess low absorption and scattering losses and be environmentally rugged and strong in order to protect the laser gain medium without compromising the light propagating through the window.
Rugged window materials such as sapphire, aluminum oxynitride (ALON), and spinel (MgAl2O4) have been evaluated as a potential replacement for fused silica windows, which are at risk of damage in some environments. These materials are rugged but tend to have an absorption coefficient at the laser wavelength of interest. Transparent spinel ceramic windows fabricated using spinel powders often show high absorption and scattering losses mainly due to the presence of chemical impurities and trapped pores, respectively. A relatively low quality window may be suitable for some applications, such as chemical and biological sensors and transparent armors. For applications where low optical loss is desired, such as exit aperture windows for high energy laser systems, however, absorption causes a cascade effect that negatively impacts the emitted laser power. When a high-energy beam is transmitted through a window material, a part of the laser energy is absorbed (usually by impurity elements that form absorption centers) and cause the window to heat. Heating, in turn, causes changes in the refractive index (n) of the window as a function of the material's thermo-optic coefficient (dn/d T). These changes result in beam distortion and loss of output power as measured by the optical path distortion (OPD) and can have a severe impact on system performance. In the worst case, a window may fail catastrophically.
Spinel is a good candidate for these applications due to its high mechanical strength, high thermal conductivity, and excellent optical transmission between 0.2-5 μm. However, spinel ceramics fabricated with powders generated by conventional methods often exhibit inhomogeneity and suffer from absorption and scattering caused by various types of intrinsic and extrinsic impurities present in the powders. See G. Villalobos et al., “Analysis of scattering sites in transparent magnesium aluminate spinel,” Cera. Eng. Sci. Proc., 26, 23 (2005). For example, the absorption of spinel fabricated with many existing powders may be above 21,000 ppm/cm in the 1-2 μm range. Therefore, high purity starting powders are desirable for preventing a potential failure of the HEL window caused by absorption.
Recently, a low absorption loss has been achieved using spinel window fabricated using powders synthesized by a purification and synthesis method described in Sanghera, et al., “Transparent Ceramics for High Energy Laser,” Optical Materials, 33, 511-518 (2011). A more cost-effective and scalable process, however, is desirable, particularly for making large size windows.
A method for purifying spinel powders is described herein which can significantly lower the absorption loss of transparent spinel ceramics while being scalable and cost-effective.